The closest itís come to a public debut was a prime-time tease during a Super Bowl ad that showed its svelte outline veiled beneath a sheet, but revealed not a glimpse of the Pentagonís most mysterious plane.

Highly classified, the program is one of the Air Forceís top priorities ó and its most expensive. The service estimates it will cost $55 billion to build as many as 100 of what it calls the Long Range Strike Bomber, which is designed to fly deep into enemy territory undetected until the mushroom cloud begins to bloom.

In the coming months, the Air Force is expected to award a contract for the next-generation bomber, which would begin flying in the mid-2020s, have the potential to fly manned or unmanned and give the military the ability to hit any target ďat any point on the globe.Ē

But thatís about all the Air Force will say about the program, which has been cloaked in a veil of secrecy rivaling the jetís stealthy ability to creep past enemy lines.

With so little known about it, there is growing concern about the systemís cost. And given the Pentagonís vast history of cost overruns on major weapons systems, experts worry that even though a contract has yet to be awarded, it is already facing the same troubling problems that have plagued other programs.

The Air Force has estimated each new bomber will cost $550 million apiece, but that figure was set in 2010 without counting for inflation and is already five years outdated. Itís also significantly less than the cost of its predecessor, the B-2 bomber, and not that much more than some high-end commercial jets. Coming in at such a low price will be difficult, if not impossible, analysts said.